1. FIELD OF THE INVENTION
This invention relates to a catalyst for sol-gel methods using metal alkoxide that can form a gel at ordinary temperatures and under neutral conditions in a short period of time, the gel forming no cracks when dried. The invention also relates to a sol-gel method that makes use of this catalyst.
2. DESCRIPTION OF THE PRIOR ART
In recent years, synthetic methods for glass make use of metal alkoxides (sol-gel methods) have received attention. In these methods, along with the hydrolysis of the metal alkoxides, polycondensation proceeds so as to produce molecules of high molecular weight. In the sol-gel method, the metal alkoxides are dissolved in alcohol, and a fixed amount of water is added, together with catalysts such as mineral acids, organic acids, inorganic bases ( or example, NaOH or KOH), ammonia, and so on; and the procedure is done at a temperature in the range of ordinary temperatures to 80.degree.C. The hydrolysis of metal alkoxides and the polycondensation reaction take place at the same time. In this method, because the glass is obtained at relatively low temperatures, it is possible to reduce costs. The method can also be used for the production of composite materials comprising metals, plastics, etc.
At first, in this sol-gel method, only one catalyst such as a mineral acid, inorganic base, ammonia, etc., is used. In a system like this in which only one catalyst is used, a long period of time, from 24 to 700 hours, is needed for gelation. Moreover, during the process of gelation, because the alcohol and water used as solvents vaporize, the hydrolyzed alkoxide brings about a non-uniform degree of polymerization, resulting in a wet gel in which there are differences in the degree of polymerization in places, which causes the formation of cracks easily in the dry gel (glass) that is obtained from the said wet gel.
A number of trials to manufacture glass of superior quality in a short period of time by the solgel method have been made. For example, Japanese Pat. Publication No. 60-27611 discloses a method for the manufacture of gels in which an acid is added to an aqueous solution of silica alkoxides so as to hydrolyze the said silica alkoxides, thereby completing the hydrolysis. Thereafter, a base is added to the reaction mixture in order to bring the pH to 5-6, resulting in a gel. In Japanese Pat. Publication No. 60-215532, a method is disclosed for the production of a gel in which acid and alkali are used to hydrolyze a metal alkoxide in separate preparations, after which the hydrolyzed preparations are mixed to make a gel. In Japanese Pat. Publication No. 57-7814, a method is disclosed in which a metal alkoxide is dissolved in a small volume of a mixture of water and alcohol, an acid is added as a catalyst to the solution, and the reaction mixture is refluxed, during which process more of the water-alcohol mixture is added, and after which a base is added to cause gelation. When these methods are used, the process of gelation is completed in a relatively short period of time (several dozens of minutes to several hours). However, the wet gel obtained is not necessarily uniform, and when the wet gel is dried to obtain a dry gel, it readily cracks. On the other hand, a method is disclosed in Japanese Pat. Publication No. 59-1161635 in which first a sol of very fine particles is made by hydrolysis of part of a solution of metal alkoxides, and this sol is added to the remaining solution of the metal alkoxides. The mixture obtained is made into a preparation of a gel containing a polycondensation product that is produced by ordinary hydrolysis and polycondensation processes. By this method, the formation of cracks is reduced, but it is not possible to obtain a gel, the uniformity of which is virtually uniform.